1. Field of the Invention
This invention relates generally to the field of Boring and Penetrating the Earth and more particularly to methods and apparatus for boring by fluid erosion.
2. Description of the Prior Art
Apparatus for hydraulic mining is well known in the art and generally employs a stream of water roughly equivalent to that ejected from a fire hose. The operation of such a system generally requires a large volume of water to be ejected at relatively low pressure, e.g. 100-200 psi. Such a system works reasonably well in cutting through earth or soft material equivalent to gravel and the like, but is totally ineffective for cutting or drilling through hard rock.
It is possible to drill through very hard rock using only a fine water jet stream if high enough pressures are applied. Apparatus of this latter type is generally described in the patent of Summers, et al., U.S. Pat. No. 4,119,160, entitled "Method and Apparatus for Water Jet Drilling of Rock". The fluid pressure commonly employed in such apparatus is on the order of magnitude of 10,000 psi and may be as high as 25,000 psi or higher.
It is common practice when water jets are used for cutting circular holes in rocks or other geological material to rotate the nozzle which requires a rotary coupling or swivel joint in the fluid conduit that supplies the cutting nozzle. Such rotary couplings must be designed to withstand such exceedingly high pressures and in use have been found to have several disadvantages. In every case, such a coupling consists of a stationary non-rotating member which channels the high pressure fluid into a rotating member. The water or other fluid must pass through a seal of some sort which prevents leakage but still allows for rotation of the nozzle head. This seal is an important component in determining the working pressure and rotational speed at which the device can operate. Frequently the seals burn out or are damaged if the coupling is subjected to a thrust load or a high rate of rotation. This has led to a situation where the rotary coupling frequently is the weak link in such a system and requires frequent maintenance to render the equipment functional. In addition to poor reliability, such devices also have shown significant pressure losses across the coupling.
The deficiencies of high pressure rotary couplings or rotary seals have been recognized by others. In the patent to Bowen, U.S. Pat. No. 3,565,191, a proposed solution was offered by providing a rotary seal at the low-pressure side and a high pressure intensifier within a housing that rotates along with the nozzle.
In addition to the poor reliability of the rotary couplings, significant pressure losses have been measured across such couplings. In an effort to make the seal more effective and thereby increase the possible operating pressure, the size of the fluid channel frequently has been reduced, resulting in a restriction to flow through the coupling. PG,4 Commercially available couplings have proven to be expensive, unreliable and very limited in hours of operation, working pressure, and flow capacity. In addition, their physical size has been a limitation in designing specific machines. The same limitation would apply to the rotary intensifier of Bowen in the patent above.